1. Field of the Invention
This invention relates generally to filter cells, and, more particularly to an improved method of making a filter cell having a sealed periphery.
2. Prior Art
Preferred type filter cells produced by the present invention are described in U.S. Pat. No. 2,788,901 to Boeddinghaus et al, particularly FIGS. 6 and 9. This reference describes filter cells in rectangular and disc form. Such cells may be used in groups (as shown in FIGS. 6 and 7 of this reference) or singularly (as shown in FIGS. 3 and 5 of this reference) to form a filter cartridge. The entire disclosure of Boeddinghaus et al is incorporated herein by reference. Probably the most outstanding problems in making such filter cells, is sealing the peripheral edges. Boeddinghaus et al describes a procedure whereby the edges of the two filter media forming the filter cell are fused by heat and pressure applied to the peripheries of the filter media. The fusing material is a thermoplastic polymer which forms a part of the filter media. Such a procedure may be highly desirable when such thermoplastic polymer forms a part of the filter media, but is useless when the filter media is comprised of a cellulosic material which may also contain, for example, inorganic particulate.
Another procedure used, is to sew the peripheral edges of the filter media together. This method of sealing the edges of the filter cell is time consuming and results in filter cells which tend to leak between the stitches.
Another method which has been used to seal the peripheral edges of a filter cell is to provide a thermoplastic separator between the filter media which extends between the edges of the filter media. When the edges of the filter media are heated under pressure the thermoplastic separator bonds the filter media together. This method is suitable for cellulosic as well as noncellulosic type filter media. The disadvantage of this method is that the seal tends to be unreliable due to delamination along the periphery of the filter cell.
Another method which has been used to seal the peripheral edges of a filter cell, is applying an extruded "L" shaped edge seal to the peripheral edge of the filter cell. The horizontal portion of the seal is placed in contact with one of the filter media on one side of the cell. The vertical portion of the seal is then formed over the other filter media on the opposite side of the cell. This results in a "U" shaped edge seal. Such a procedure for sealing the periperhal edges of a filter cell, has not been successful for the molded edge seal is expensive, the vertical portion of the "L" shaped edge seal tends to spring back when pressed against the upper filter media and also tends to stick to the press which bends it over the upper filter media.
Another method used to seal the periphery of a filter cell is to dip the peripheral edges of the filter media in a thermosetting plastic, e.g. melamine formaldehyde, and then set the plastic with heat. This method is undesirable for it produces a filter cell having measurable extractables, a property which is undesirable in the use of such filters for foods, pharmaceuticals and parenteral fluids. Additionally, such a method makes the edges of the filter cell highly brittle causing, for example, radial cracks to form. Additionally, process parameters are critical and the fumes, for example from melamine formaldehyde, are undesirable pollutants.
Another method for sealing the peripheral edges of a filter cell uses an extruded polypropylene seal which is placed around the flanges of the filter cell. This method reduces extractables, but such a a seal is expensive and time consuming to properly place on the flanges of the filter media.
Still another method of sealing the peripheral edges of a filter cell is dipping the edges in a thermoplastic, for example, polyvinyl chloride, which has a plasticizer admixed therewith, and then allowing the filter cell to dry. This method produces a filter cell having high extraction levels making it unsuitable for critical food and pharmaceutical applications.